273 Integer to English Words 整数转换英文表示

将非负整数转换为其对应的英文表示，给定的输入是保证小于 2³¹ - 1 的。
示例：
123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

详见：https://leetcode.com/problems/integer-to-english-words/description/

Java实现：

每三位数一读。每读三位数的时候在三位数后面加上单位（Thousand, Million, or Billion）即可。其中的关键点是当读取的三位数是“0”，那么后面的单位就要舍弃。比如1000000，读第二个三位数是“000”，那么对应的单位Thousand是要舍弃的，否则就会变成One Million Thousand的错误结果。

class Solution {
    //全局变量先把要用的英文存起来
    String[] units = {"", " Thousand", " Million", " Billion"};
    String[] num0To9 = {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    String[] num10To19 = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    String[] num10To90 = {"Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

    public String numberToWords(int num) {
        if (num == 0){
            return "Zero";
        }
        String res = "";
        int count = 0; //记录当前三位数下后面跟的单位
        while (num > 0) {
            String tmp = "";
            tmp = units[count]; //记录当前三位数下后面跟的单位
            int cur = num % 1000; //每三位一读，从后往前
            String pre = convert(cur); //转化当前数字最后的三位数
            if (pre == ""){
                tmp = ""; //如果是"000"，那么就等于什么都没发生，舍弃单位
            }else{
                tmp = convert(cur) + tmp; //否则结合结果和单位
            }
            if (res.length() != 0 && res.charAt(0) != ' '){ //处理一下加上单位的空格情况
                res = tmp + " " + res;
            }else{
                res = tmp + res;
            }
            num = (num - num % 1000) / 1000; //处理往前三位数
            count++;
        }
        return res;
    }
    //转化任意三位数
    public String convert(int num) {
        if (num == 0){
            return "";
        }
        if (num < 10){
            return num0To9[num];
        }else if (num >= 10 && num <= 19){
            return num10To19[num - 10];
        }else if (num >= 20 && num <= 99) {
            if (num % 10 == 0){
                return num10To90[num / 10 - 1];
            }else{
                String s1 = num0To9[num%10];
                String s2 = num10To90[num/10 - 1];
                return s2 + " " + s1;
            }
        }
        else {
            if (num % 100 == 0){
                return num0To9[num / 100] + " Hundred";
            }else {
                String tmp = convert(num % 100);
                return convert(num - num % 100) + " " + tmp;
            }
        }
    }
}

C++实现：

class Solution {
public:
    string numberToWords(int num) {
        string res = convertHundred(num % 1000);
        vector<string> v = {"Thousand", "Million", "Billion"};
        for (int i = 0; i < 3; ++i)
        {
            num /= 1000;
            res = num % 1000 ? convertHundred(num % 1000) + " " + v[i] + " " + res : res;
        }
        while (res.back() == ' ')
        {
            res.pop_back();
        }
        return res.empty() ? "Zero" : res;
    }
    string convertHundred(int num) {
        vector<string> v1 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        vector<string> v2 = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        string res;
        int a = num / 100, b = num % 100, c = num % 10;
        res = b < 20 ? v1[b] : v2[b / 10] + (c ? " " + v1[c] : "");
        if (a > 0)
        {
            res = v1[a] + " Hundred" + (b ? " " + res : "");
        }
        return res;
    }
};

参考：https://www.cnblogs.com/grandyang/p/4772780.html